1. Field of the Invention
The present invention generally relates to hull structures for water-borne vessels and, more particularly, to double hull structures having improved resistance to damage and cargo containment.
2. Description of the Prior Art
Double hull designs for hull structures for water-borne vessels are known and have been used to achieve several design purposes such as provision of buoyancy and increasing hull rigidity. Double-hull designs became particularly prominent when the Great Eastern, the largest vessel to have been built at that time incorporated a double-hull design. More recently, however, double-hull designs have been considered for the purpose of improving cargo retention for flowable cargoes such as liquid oil or pulverent materials such as grain or coal which are loaded directly into the hull of the vessel for transport and are not otherwise contained. The theory of such a double-hull construction is that, upon grounding or collision with another vessel which may pierce the outer hull, the inner hull will be more likely to remain intact to prevent sinking of the vessel and to contain the cargo, preventing spillage into the environment.
At present, there is an average annual spillage of 9,000 tons of crude oil and petroleum products in U.S. waters. Large spills of 30,000 tons or more, however, while constituting only about 3% of events in which spillage occurs, accounts for nearly 95% of the quantity of accidental spillage in U.S. waters. This spillage, particularly from large spills represents a major economic cost which has only recently been considered as an aspect of efficiency of hull performance. In particular, single hull designs which were previously considered to be more "efficient" have reduced allowances for deterioration and accidents such as groundings and collisions to a significant degree. Such vessels will cause spillage whenever the single hull is pierced.
However, even double hull structures, as designed and fabricated in the past, have numerous drawbacks. A double hull design of steel will necessarily increase the construction and material costs of vessel production, especially due to the labor intensive joining requirements of welding of the hulls as compared with single hull designs. Even though hull plate thickness may be halved in a double hull design to reduce material cost, the labor intensiveness of joining sections greatly outweighs any potential savings. Increased weight due to the joining of hull sections also implies a substantial increase in empty hull displacement, increasing wetted area of the hull for a given cargo mass or volume and requiring increased motive power and fuel consumption for the vessel. Further, the space between the hulls is not readily accessible for inspection and maintenance (particularly since double hull designs are typically compartmentalized), increasing the costs of vessel operations. This is particularly important in double hull designs fabricated from steel since weldments for joining the steel plates of the hull will typically not be fully watertight and will form a source of slow leakage. This exposure to sea water in inaccessible locations forms a serious problem of corrosion and corrosion prevention. For this reason, hull deterioration which may affect the sea-worthiness of the vessel are not readily detectable.
Perhaps more importantly, to assure good performance of the outer hull as protection for the inner hull, a rule-of-thumb has been developed that the outer hull should be separated from the inner hull by about 1/10th to 1/15th or 6.7% to 10% of the beam of the vessel for conventional metallic design. This separation has been determined from the potential shock absorbing qualities of double hull designs. It should be noted in this regard that traditional structural materials such as steel, when subjected to a force will yield elastically over a substantial dimension before inelastically yielding. Little energy dissipation occurs during the elastic deformation because of the high stiffness of the material, whereby the forces are passed directly to the inner hull and the only benefit to be derived in protection from grounding or collision damage during elastic deformation is the spreading of the forces which are encountered. Therefore, a substantial distance must be provided so that the point of inelastic deformation can be reached under such circumstances and energy can, in fact be dissipated to protect the inner hull.
This rule-of-thumb has enormous economic consequences for several reasons. For example, the beam of a large tanker may run to well in excess of 100 feet and a separation between inner and outer hulls would thus be in excess of 6.5 feet. This increases the beam of the vessel by over 13%, greatly increasing wetted surface and frontal area of the vessel. Further, the volume of unusable space greatly diminishes the cargo carrying capacity for a given hull displacement. Additionally, structure must be provided between the inner and outer hulls which increases the material cost and weight and labor in the fabrication of the hull and reduces the efficiency of the vessel.
These factors are especially aggravated in the case of barges used on intercoastal waterways. Such barges often have beams (e.g. the width of the vessel) in excess of 200 feet. While double hull constructions may possibly be cost-effective in ocean-going tankers when the actuarial costs of damage from cargo spillage and other non-operational costs (such as adverse publicity incident to spillage into the environment) are considered, double hull structures for such barges is considered prohibitive.
Over the operational lifetimes of vessels now in service (e.g. over the past approximately thirty years), costs incident to cargo spillage have generally not been considered in the design of cargo vessels. Consequently, virtually all cargo vessels now in service are of single hull design. While these vessels represent a substantial economic investment, they also represent a substantial hazard to the environment and a potential major liability to owners and operators. Therefore insurance rates for such vessels have greatly increased in recent years. On the other hand, replacement of such vessels with double hull vessels represents an extremely large cost both in terms of the cost of new vessels and the loss of usable lifetime of the vessels replaced. Insurance rate savings are not fully realized, in any event, due to the difficulty of inspection and the determination of seaworthiness of double hull structures after they are placed in service. Operational costs would also be increased due to both the reduced cargo capacity for a given hull displacement and for amortization of the cost of new hull construction and the loss of useful lifetime of replaced single hull vessels.
In view of the above considerations, particularly under the present atmosphere of environmental sensitivity and regulation, double hull designs fabricated from metal presents an economically and technically inadequate solution to the problems of damage limitation and cargo containment during groundings and collisions of large vessels.